1. Field of the Invention
This invention relates to a method and apparatus for forming electric connections between predetermined bonding pads of semiconductor electronic devices and respective electric contact leads formed on a frame in which the semiconductor electronic devices have been arranged.
2. Discussion of the Related Art
In the manufacture of semiconductor electronic devices, the electronic device proper (i.e., a piece of suitably doped silicon, sometimes referred to as a die) is provided with leads for setting up electric contacts. Each of the leads is connected electrically to a definite area of the silicon. To this end, the silicon piece is placed securely onto a very thin metal frame, which frame is, among others, formed with sections that are eventually to be fashioned (as by appropriate blanking) into the leads of the device. The electric connection between each of the leads and its corresponding silicon pad is made by depositing a wire conductor and bonding it to both the silicon and the lead. Typically, the bonding wire is made of gold, and bonding is a ultrasonic process.
The bonding operation, although conceptually simple, is made highly critical by the minute size of the connections to be formed and their large number; a typical semiconductor electronic device includes a few hundred connections spread over the perimeter of a square measuring less than two cm along each side. The acceptable margin for error is almost nil, because a single faulty bond out of the hundreds made on one device would turn the whole device into a reject. Further, the time to create all the bonds should be the shortest possible if large production volumes are sought.
Bonding is conventionally performed by a head mounted on an X-Y table, which is a mechanism that can be moved in a plane controllably along two spatial coordinates, X and Y. Tables are known which, while ensuring the necessary high degree of accuracy, can work at extremely fast rates, on the order of 100-200 ms per connection, including bonding to the silicon, depositing the wire, bonding to the lead, and cutting the wire. Tables of this type will be referred to as very high precision tables hereinafter.
Once the connections have been made on a semiconductor electronic device, the frame is indexed forward far enough to bring the next device on which connections are to be made within reach of the bonding head. The metal frame accommodates a set of identical electronic devices lined up one behind another. The frame may either be a definite length or in the form of a flexible strip, stored on a reel that is fed forward continuously and cut at intervals. Upon completion of the connections on a given number of the devices, the whole batch is transferred to the next processing step, wherein an insulating resin is poured over the devices to encapsulate the leads. The resin pouring step takes a fairly long time, for both pouring and curing the material, and is often found to be the bottleneck of the whole process. Therefore, frames have been developed wherein the devices can be lined up into two columns instead of one.
If the device size is very small (less than one centimeter), conventional machines can handle pairs of side-by-side devices without undue difficulty. With larger size devices, however, the processing head must be slowed down because the larger working area to be spanned may degrade accuracy to an unacceptable extent. This reduced speed frequently is enough to thwart any advantage gained in the pouring step, and accordingly many conventional systems do not use the paired device setup.